1. Field of the Invention
The present invention relates to a data conversion device that is used to synthesize digital data composed of two or more components, such as a DTMF (dual-tone multi-frequency) signal used in a push-button (i.e. tone-dialing) telephone, into a single analog signal.
2. Description of the Prior Art
Conventional examples of such data conversion devices are shown in FIGS. 1 and 2. FIGS. 1 and 2 are block diagrams of conventional data conversion devices that synthesize 1 kHz digital data 20, 10 kHz digital data 21, and 100 kHz digital data 22 into an analog signal OUT.
In the conventional data conversion device shown in FIG. 1, the 1 kHz digital data 20 is converted into an analog signal by a D/A (digital-to-analog) converter 23. The analog signal output from the D/A converter 23 is then amplified by an amplifier circuit 26 by a predetermined amplification factor and fed to an adder circuit 29. The 10 kHz digital data 21 is converted into an analog signal by a D/A converter 24. The analog signal output from the D/A converter 24 is then amplified by an amplifier circuit 27 by a predetermined amplification factor and fed to the adder circuit 29. The 100 kHz digital data 22 is converted into an analog signal by a D/A converter 25. The analog signal output from the D/A converter 25 is then amplified by an amplifier circuit 28 by a predetermined amplification factor and fed to the adder circuit 29. The adder circuit 29, by adding together the individual analog signals output from the amplifier circuits 26, 27, and 28, synthesizes those signals into a single analog signal OUT.
On the other hand, in the conventional data conversion device shown in FIG. 2, the 1 kHz digital data 20 is, while it is digital data, multiplied by a multiplier 30 so as to attain a predetermined amplification factor, and is then fed to an N-bit adder 33. The 10 kHz digital data 21 is multiplied by a multiplier 31 so as to attain a predetermined amplification factor, and is then fed to the N-bit adder 33. The 100 kHz digital data 22 is multiplied by a multiplier 32 so as to attain a predetermined amplification factor, and is then fed to the N-bit adder 33. The N-bit adder 33 adds together the digital data output from the multipliers 30, 31, and 32, and feeds the sum to a D/A converter 34. The D/A converter 34 converts the digital data fed thereto into an analog signal so as to output a single analog signal OUT. In this way, either of the data conversion devices shown in FIGS. 1 and 2 synthesizes digital data composed of three components 20, 21, and 22 of different frequencies into a single analog signal, while maintaining the frequencies of the components, and outputs the obtained single analog signal to a communications network, bus, or the like.
However, the conventional data conversion devices shown in FIGS. 1 and 2 have the following disadvantages. When digital data composed of a plurality of components is synthesized, the data conversion device shown in FIG. 1 requires amplifier circuits 26, 27, and 28 to attain a predetermined amplification factor for each signal component, and the data conversion device shown in FIG. 2 requires multipliers 30, 31, and 32 one for each signal component. Thus, these conventional data conversion devices require either large-scale hardware if their function is realized on a hardware basis or large-scale software if their function is realized on a software basis.